When customers inquire about air filters, regardless of their filtration efficiency or purpose, my usual response is: “Please provide the filter size you need, and if you can send air flow also, that would be better.” This is also the first question I ask customers. Generally, the efficiency and size of the filter are essential information that manufacturers must know, and it’s the most basic information compared to airflow. Customers may only care about the filter’s model number because a label indicates the model number.
However, different manufacturers may not know the model number of another manufacturer’s filter. Therefore, having clear information about the filter size and efficiency in the form is the most effective way.
Of course, customers may have other requirements for the air filter, such as frame material, odor absorption, or weight requirements. But these are topics for further discussion.
I usually define the filter’s size and efficiency under the air filter’s specifications because these two parameters are equally important.
Regarding the filter’s efficiency, different countries have corresponding standards to define it, and there are many similar articles online so I won’t go into detail here. Perhaps in another article, I will write about filter efficiency.
In this article, I want to talk about the size and airflow of the air filter. I have summarized six aspects and will focus on discussing the size and airflow of filters in general ventilation filtration systems.
Pocket filter: 592mm * 592mm
Bag filters are the most commonly used filter type in central air conditioning and centralized ventilation systems. In developed countries, these filters have a nominal size of 610mm * 610mm, corresponding to an actual outer frame size of 592mm * 592mm.
The ventilation system filter section comprises several units measuring 610mm * 610mm. Sometimes, to fill the entire filter section, filters with dimensions of 305mm * 610mm and 508mm * 610mm are placed along the edges of the filter section.
In the past, some developing countries did not have uniform size standards for bag filters. Manufacturers and designers operated independently, resulting in various filter sizes. This created design challenges, inconvenience for users, and low production efficiency for filter manufacturers.
With the influx of imported central air conditioning systems, the 24-inch size gradually followed. People slowly became accustomed to this size, and although it was never officially mandated, in recent years, the 24-inch size has become the mainstream size for filters in many countries.
Other types of filters — still stay 24 inches
Besides bag filters, various other filters have been developed in ventilation systems. Only a few manufacturers adopt innovative specifications when determining their sizes, while most manufacturers and users stick to the standard 24 inches. The installation methods for filters are also generally similar across the board.
In the market, many products serve as alternatives to bag filters, such as V/W-type filters and box filters with installation frames. To compete with bag filters, many of these alternatives can be interchanged with bag filters, requiring the same installation methods and frame sizes. Some alternative products may only have minor differences in size, such as 595 mm, 593 mm, or 597 mm.
Most other types of filters are also 24 inches, with actual sizes varying depending on the filter type and manufacturer. For example, the frame size of flat panel filters is mostly 595 mm, while the outer frame size of box filters is 592 mm, 595 mm, or 610 mm.
Users prefer universality
I will describe two scenarios in this part.
A user consumes over 6,000 general ventilation filters annually, all of which come in only four efficiency specifications and two installation size specifications. This benefits maintenance, spare parts storage, and procurement work.
Most air filter manufacturers can produce these filters in bulk. With centralized bulk production, standardized specifications, reliable supply, guaranteed quality, and competitive pricing are ensured.
Another user has over 300 air filter installations, with as many as 22 different specifications, which brings a lot of trouble to daily maintenance and spare parts management. Suppliers are unwilling to lower prices, delivery times are long, quality is not guaranteed, and switching suppliers is not easy.
Using air filters with the same specifications brings many benefits.
HEPA filter sizes
Since the emergence of HEPA filters in the 1940s, the standard frame width for high-efficiency filters in developed countries has consistently been 610mm (24 inches). In the 1980s, imported filters accompanied complete equipment sets in other countries. By the 1990s, the 610mm size became prevalent in many countries.
Derived sizes from the 610mm include 203mm, 305mm, 762mm, 915mm, 1219mm, 1524mm, and 1829mm (8 inches, 12 inches, 30 inches, 36 inches, 48 inches, 60 inches, 72 inches).
The traditional 610 series with separator filters comes in 292mm and 150mm thicknesses. The latter is primarily used for terminal filters in cleanrooms. Nowadays, panel non-separator HEPA filters have replaced traditional separator filters in cleanrooms, with thicknesses typically ranging from 65 to 100mm.
The thickness of HEPA filters used in HVAC systems remains 292mm. They are available in two width sizes: 610mm and 305mm. Some filters are designed with 610mm and 305mm as installation modules, with actual dimensions slightly smaller.
Some uncommon HEPA filter sizes
In 1965, some countries introduced their own HEPA filters with dimensions of 484mm*484mm*220mm. At that time, the designers set the installation module as 500mm*500mm, considering the need to reserve space for surrounding suspension rods, thus determining the filter section as 484mm*484mm.
630 Series
In these countries, another popular size for HEPA filters is 630mm*630mm*220mm. Similar to the background of the 484mm size, the designers set the installation module as 650mm, subtracting the installation clearance, resulting in 630mm. The derived sizes for 630mm are 315mm, 945mm, and 1260mm.
HEPA filter sizes and air volume
Conventionally, the rated air volume for a 484mm x 484mm x 220mm filter is 1000m3/h; for a 630mm x 630mm x 220mm filter is 1500m3/h, and for a 320mm x 320mm x 260mm filter is 500m3/h. These rated air volumes are the maximum allowable, but people often discount these rated values in practice.
The airflow data for the 610mm series filters is reliable and matches the actual conditions.
For clean rooms with unidirectional airflow, the filter air velocity typically ranges from 0.2 to 0.5m/s, and filter testing is conducted under conditions of 0.45m/s. In this case, the filter specification refers to the rated air velocity rather than the rated airflow.
Air volume algorithm for standard sizes of HEPA air filters
The airflow of HEPA air filters is generally determined based on the airflow velocity and the size of the filter. Typically, the rated airflow velocity for HEPA filters ranges from 0.3 to 0.6 m/s, and a common value is often chosen, such as 0.45 m/s. For example, let’s consider the size of a deep pleat hepa filter known for its 1000 airflow capacity:
Using the standard size of 484x484x220mm for the HEPA filter with separator.
The rated airflow is 1000 M3/H, with an initial resistance of 200 Pa.
Based on this information, we can calculate the airflow for the 900x600x370mm HEPA filter as below:
1. The filter paper has a pleat count of 28 pleats/10cm for a 484x484x220mm HEPA filter.
2. The calculation for the effective area of the filter paper is as follows: 0.475×0.475×280×0.2×2=25.27 m2, where 0.475 represents the effective length of the filter paper, 280 represents the number of pleats for 1 meter of filter paper folded in double layers, 0.2 means the actual height of the filter paper.
3. The air velocity through the HEPA filter, calculated as 1000/3600/25.27, is 0.011 m/s. This means that when the air velocity through the filter paper is 0.011 m/s, the initial resistance of the HEPA filter is 200 Pa.
Next, I will calculate the airflow for the HEPA filter with dimensions 900x600x370mm:
0.88×0.58×280×0.35×2×0.011×3600=3961m3/h
Where:
- 0.88 is the effective length of the filter paper.
- 0.58 is the effective width of the filter paper.
- 280 is the number of folds for 1 meter of standard filter paper.
- 0.35 is the actual height of the filter paper.
- 2 accounts for the double layer.
- 0.011 is the air velocity through the filter paper in meters per second.
Last summary
The rated airflow of a filter is the maximum airflow it can handle, determined by the area of the filter material (not the air filter itself). If the airflow velocity through the filter material is consistent, a larger filter material area allows for greater airflow. Currently, the rated airflow of filters with the same structure depends on their size.
